In a conventional multi-coin validator, coins pass along a path past at least one sensor coil energised to produce an inductive coupling with the coin. The degree of interaction between the coil and the coin can be used to discriminate between different coin denominations and fraudulent coins. An example of such a validator is described in our co-pending application PCT/GB92/00791.
After passing the inductive sensor(s) the coin passes towards a solenoid operated accept gate. If, as a result of the inductive test, the coin is determined to be of acceptable denomination, the accept gate is opened and the coin passes along an accept path. Alternatively, if the coin is determined to have non-acceptable characteristics, the gate remains closed and the coin is diverted to a reject path. Operation of the gate is controlled by a microprocessor in dependence upon the output of the sensor(s). In order positively to confirm that an acceptable coin has passed the accept gate into the accept path, a sensor is included in the accept path, which provides an output to the microprocessor so that, for example, the microprocessor can monitor the credit accumulated through the accept path.
This post-acceptance sensor in the accept path has hitherto been constituted by a further inductive sensor but recently, proposals have been made to use optical sensing arrangements in the accept path. It has previously been proposed to use a pair of infra-red sources each with an associated phototransistor mounted in a common wall of the accept path. When a coin passes along the accept path, its side surface reflects infra-red radiation from at least one of the sources to the detector(s) in order to enable the coin to be detected. Source-detector pairs are used in order to provide sensitivity over the entire width of the accept path which may be significantly wider than the diameter of the coin. However, on occasions, the optical sensing arrangement may not detect an acceptable coin, particularly small coins, due mainly to the angle at which the coins fall as they pass the sensors or the dullness of the coin's surface. Whilst it would be possible to increase the sensitivity of the system by increasing the power of the emitted infra-red radiation, a problem arises in that with increased power, significant levels of radiation are reflected from the opposite side wall of the accept path, towards the detectors, which degrades their performance, since the difference between the levels of radiation received by the detectors in the presence and absence of a coin is reduced by the increased level of reflection from the opposite side wall.